get_multiple doesn't document result order

[ecton]

The implementation of get_multiple does not document that the returned list's order is not guaranteed. Additionally, we might want to add another function such as get_multiple_ordered that performs the additional step of sorting the results to match the input request, minus any missing items.

Originally inspired by a question by @martpie in https://github.com/khonsulabs/bonsaidb/issues/237#issuecomment-1987342741

[martpie]

I can maybe work on a PR if you don't have the bandwidth :)

[ecton]

I'm not currently focused on this project but don't mind tackling this once I get back to BonsaiDb. But I'm also more than happy to accept a PR!